In the field of processing semiconductor memory devices, through-mask electroplating is a method for forming the noble metal electrodes expected to be required for devices incorporating high permittivity dielectric or ferroelectric materials. Such electrode plating is done selectively with the plated deposits being grown in defined areas of a conductive plating base layer on a substrate, which areas have been defined by lithography. While, for consistency of description, both the background of the invention and the description of the invention will be detailed with specific reference to through-mask electroplating of electrodes for semiconductor memory devices, it is not intended that the applicability of the invention be limited solely to those processes. It will be apparent to one having skill in the art that the current invention has applicability for multilayer metal deposition for other applications as well.
Through-mask plating of noble metals (and for non-noble metals as applicable), is illustrated in FIGS. 1(a) through 1(e). Reference is made to Assignee's application entitled "Plating of Noble Metal Electrodes for DRAM and FRAM", of Andricacos, et al, which issued as U.S. Pat. No. 5,789,320 on Aug. 4, 1998, the teachings of which are herein incorporated by reference. Through-mask plating is generally conducted in accordance with a process flow that begins with the deposition of a blanket layer of a conductive plating base, 1 of FIG. 1(a), deposited over the substrate, 2, on which the electrode deposits are to be grown. The plating base layer is typically thin, of the order of 20-100 nm. One or more layers may be deposited on substrate 2 prior to deposition of the plating base, to improve adhesion or to act as barriers to interlayer reactions. After the blanket layer of the plating base has been deposited, a layer of mask material, 3, is deposited on the plating base layer, followed by patterning of the mask material to expose areas of the plating base layer where plating is desired. As shown in FIG. 1(b), the mask material remains over the regions of the surface which are not to be plated. The thickness of the mask material should be greater than or equal to the thickness of the desired electrodeposit. The mask material, such as organic photoresists or diamond-like carbon (DLC), should be insulating and compatible with the plating solution. Next, the entire patterned surface is exposed to a plating step, whereby the chosen electrode plating material, 4, is selectively grown on the exposed conductive regions of the sample, as shown in FIG. 1(c). After electroplating, the mask material is removed from the areas of the plating base layer which have not been plated, leaving the structure illustrated in FIG. 1(d); and, finally the plating base is removed from those areas having no electrodeposited material, yielding the structure of FIG. 1(e).
Plating base removal is typically done by etching. The etchant and etch method may be selective or non-selective with respect to the electrode plating material, with selective etches clearly being preferred, so as to preserve the plated electrode during the plating base removal process. A complication frequently encountered is that the materials chosen for the plating base and the plated electrode are so similar that they respond similarly to a given etchant or etch process.
Furthermore, the etching process may be anisotropic (e.g, including so-called dry etching processes such as reactive ion etching) or isotropic (e.g., including so-called "wet" etching such as a bath of chemical solution). Wet etches are generally not preferred for plating base removal when fine features are present, since the wet isotropic etches do give rise to the aforementioned risk of removal of some or all of the plating base under the electrode. What is typically encountered with isotropic etching for plating base removal is partial removal of the plating base at the edges of the plated electrode feature, an effect known as "undercutting," as illustrated in the structure of FIG. 2(a). The amount of undercutting is dependent upon the duration of the etch process, with the minimum amount of undercut generally about equal to the thickness of the plating base which is being removed. If the lateral dimensions of the plated feature are large compared to the plating base thickness, an undercut of that thickness on each side of the feature may be tolerable. However, wet etching may be problematic for the sub-micron dimension electrode structures of interest for memory device applications, even with plating base thicknesses of 30-50 nm, since plating base undercutting may expose underlying contacts or degrade electrode-to-substrate adhesion.
As a result of the foregoing concerns, anisotropic (typically dry) etch processes are preferred for plating base removal from a device structure having fine-featured electrodeposits. If the dry etch is selective, it will only remove the plating base which had been covered by the mask material. If the dry etch is non-selective, it will remove the plating base and a small amount of the electrodeposit (i.e., the electrode plating material) at the top surface of the plated structure. Since the thickness of the electrodeposit is much greater than the thickness of the plating base layer, the sacrifice of a small amount of the electrodeposit from the top surface is acceptable. A disadvantage of using a physical dry etch process, such as rf-sputtering or ion beam sputtering, is that the surfaces of the plated electrode structures may become contaminated with residue from the etching process, including redeposits of sputtered material comprised of the plating base material and the underlying surface material, as shown in the structure of FIG. 2(b). This results in degradation of the electrical properties of the electrode structure and of a subsequently-deposited cell dielectric.
What is desired, therefore, is a selective etching process which is not subject to any of the foregoing disadvantages.
It is therefore an objective of this invention to provide a selective etching process for the removal of a conductive noble metal plating base material from the regions between patterned electrodeposits of a noble metal material which differs from the noble metal plating base material.
It is another objective of the invention to improve the properties of layered structures comprised of a plurality of layers of two or more different conducting materials.
It is still another objective of the invention to improve the properties of layered electrodeposit-on-plating-base structures intended for microelectronic or micromechanical applications.
Yet another objective of the invention is to provide a method and materials for a through-mask electroplating process which permits the plating base to be removed from the regions between the patterned electrodeposits by a wet chemical etch which is selective and anisotropic.
Still another objective is to provide a multilayer metal structure having improved oxidation resistance.